Vascular plants were the next evolutionary step

The Devonian Period: First forests

The big picture: Vascular plants now dominate Earth



  • Bryophytes ruled for <100 million years
    • vascular plants show up ~425mya


  • Plants evolved vascular tissues
    • grow taller than bryophytes
    • connects leaves & roots


  • First vascular plants lacked seeds
    • sperm still need water to swim
    • ferns and lycophytes


  • Fern and lycophyte diversity now diminished

Give thanks to extinct seedless vascular plants: Coal

Vascular system: the basics




  • Xylem = water conducting (one way)
  • Phloem = sugar conducting (multi-directional)


  • Conducting cells provide biomechanical support
    • lignin in cell walls
    • does not rot easily
    • does not collapse easily
    • think bundles of rigid straws or pipes


  • Vascular elements move liquid under tension









Vascular elements evolved independently several times

Evolution of roots

  • Vascular tissues belowground
    • extension of ancient stems
    • bryophytes have no ‘true’ roots


  • Absorb water and nutrients
    • active explorers


  • Anchor the plant in place
    • support vertical growth


  • Fossils show roots in lycophytes ~ 400mya
    • convergent evolution in ferns

Evolution of leaves



  • Photosynthetic organ on plants
    • maximize surface area


  • Microphylls: single unbranched vein
    • unique to lycophytes


  • Megaphylls: leaves with branched veins
    • majority of vascular plants


  • Why does this matter?

Evolution of leaves: sporophylls


  • Sporophyll: modified leaf with sporangium
    • vastly different looks
    • Trophophyll: vegetative leaf


  • Sporagnium: spore container
    • ferns: clusters of sporagnia called ‘sori’
    • lycophytes: stobilus holds many sporagnia (like a cone)
    • angiosperms: structures in flowers


  • Spore mother cell: cell inside sporagnium
    • undergoes meiosis to make spores


  • What is the ploidy of each cell type in this picture?

Lycophytes: First vascular plants



  • Independent, branched sporophyte


  • Lignified vascular tissue, tracheids & roots
    • tracheid = special xylem cell


  • Leaves are microphylls
    • Some with strobili (evolved multiple times)


  • Some with heterospory (Selaginella/Isoetes)
    • male (micro-) and female (mega-) spores

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous
    • whisk ferns do not have roots


  • Homospory = spores that germinate to produce bisexual (both male and female) gametophytes


  • Leaves are megaphylls
    • whisk ferns lost leaf (enation)
    • Equistem has simplified megaphylls

Ferns


  • Whisk Ferns, Equisetum & Ferns
    • mostly homosporous
    • whisk ferns do not have roots


  • Leaves are megaphylls
    • whisk ferns lost leaf (enation)
    • Equistem has simplified megaphylls


  • New Gaga genus with 19 species
    • “We wanted to name this genus for Lady Gaga because of her fervent defense of equality and individual expression” Pryer @ Duke University

Homosphorous life cycle (mostly) with dominant sporophytes

The consequences of homospory




  • One spore → one gametophyte → bisexual gametophyte
    • everything is haploid (1N)


  • Produces both sperm and egg via mitosis


  • A lot of self fertilization
    • What is good/bad about this?

How spore-bearing plants avoid self-fertilization…




  • Turn other gametophytes male
    • sperm from everywhere!


  • Archegonia (F) and Antheridia (M) mature at different times

Seedless vascular plants have free living separate generations


  • Sporophyte: The fern you see in the woods
    • dominant stage


  • Gametophye: independent haploid generation
    • start out female or bisexual


  • Derived Trait - remember how a moss looks!


  • What does natural selection favor diploid dominant lifecycles?